1. Field of the Invention
The present invention relates to a light scanning unit. More particularly, the present invention relates to a light scanning unit suitable for an exposure device for an electrophotographic apparatus, such as a color printer having a plurality of exposed objects.
2. Description of the Related Art
An electrophotographic exposure device can be categorized into a devices that use a laser diode and devices that use a light emitting diode (LED). Devices that use an LED usually perform an exposure process by mapping one LED on 1 dot of a pixel of an image to be recorded for an exposed object. In general, devices using the LED use a light source called an LED head in which a plurality of LEDs are arranged.
The light source is configured to arrange an LED chip in which a plurality of LEDs are formed on a substrate and to form a plurality of LED arrays. Japanese Patent Publication No. Hei 10-035011 (published on Feb. 10, 1998) entitled “Light Emitting Diode Array and Fabrication thereof”, which is incorporated herein by reference discloses this type of configuration. The disclosed apparatus condenses light onto an image-formed surface by arranging an optical system between an LED and an exposed surface of an exposed object.
In addition, devices using a laser diode scan a laser beam in a main scanning direction on an exposed surface of an exposed object using a light scanning unit.
Devices using a laser diode use an F-θ lens to maintain the same scanning speed and beam shape on an exposed surface. Japanese Patent Publication No. Hei 09-096769 (published on Apr. 8, 1997) entitled “Method and Device for adjusting Optical Axis of Optical Scanner and Optical Scanner”, which is incorporated herein by reference discloses this type of configuration.
In the disclosed apparatus, a laser diode emits a laser beam comprising diffused light, and the laser beam is collimated by a collimator lens. The shape of the collimated laser beam is restricted by a slit, and the laser beam is condensed by a cylinder lens in a subscanning direction on a reflective surface of a polygonal rotating mirror which is a light scanning unit. Subsequently, light scanned in a main scanning direction on the polygonal rotating mirror is condensed by an F-θ lens (or lens group) on the exposed surface of an exposed object and is scanned at a uniform speed.
An electrophotographic exposure device using the LED can be reduced in size. However, the following problems must be resolved in order to reduce the size of the device using the LED. First, a plurality of LED chips need to be precisely arranged on a substrate. Second, the circuit of conventional devices is too complex. Third, due to the characteristics of an optical system, the distance from each LED chip to an exposed surface of an exposed object should be precise. Fourth, the quantity and degree of nonuniformity of light between LED chips needs to be corrected.
In addition, in order to scan a laser diode light source using a polygonal rotating mirror onto an exposed surface, the amount of light projected onto the exposed surface is small, the circuit is simple in structure, the depth of focus is deep. Thus, distance precision with respect to an exposed surface is comparatively low.
Recently, electrophotographic color printers have rapidly been developed. The electrophotographic color printer forms an image four times, compared to a conventional black and white printer, so as to condense an image having four colors, such as cyan, magenta, yellow, and black, into one image.
An electrophotographic color printer can be categorized into two types. Single pass electrophotographic color printers and multi-pass electrophotographic color printers. Single pass electrophotographic color printers mount one exposure device in one drum, perform a development process using a four-color developer, superimpose an image on an intermediate transfer body, and transfer the superimposed image onto a sheet of paper.
Multi-pass electrophotographic color printers mount four developers and four exposure devices, and four photosensitive bodies which is mechanically like having four conventional black and white printers superimposed on one another.
In the single pass electrophotographic color printer, an output speed is reduced to ¼ the normal speed, to superimpose an image four times. The single pass electrophotographic color printer has a low speed, and a complex mechanism for moving a developer. In addition, the single pass electrophotographic color printer can configure a photosensitive drum and an exposure device as a single body.
The printing speed of the multi-pass electrophotographic color printer is fast. However, the multi-pass electrophotographic color printer requires four exposure devices and four photosensitive bodies. Thus, the structure becomes complex.
However, a composite light source, which forms a laser diode emits a plurality of light beams having different wavelengths on the same substrate, and is widely uses in a variety of devices. In particular, in an optical pickup, a composite light source having a plurality of laser diodes for emitting light beams having different wavelengths is used as a light source, and reading and recording of data is performed on different media in different optical systems.
Accordingly, as shown in FIGS. 1 and 2, a plurality of laser diode chips 60 and 62 are formed on a substrate 50 integrated into the semiconductor process to reduce manufacturing costs. Here, FIG. 1 is a front view of a composite light source, and FIG. 2 is a side view of FIG. 1, and reference numerals 60A and 60B are excitation light-emitting portions.
In the optical pickup using the composite light source, only one recording medium can be mounted. Thus, after the type of recording medium is recognized, a laser diode for emitting a light beam having a wavelength used to reproduce data recorded on the recording medium is consecutively emitted, and the lights of a laser diode for emitting a light beam having a wavelength that is not used to reproduce data recorded on the recording medium are extinguished.
Meanwhile, when the above-described composite light source is used in an optical printer, information on different images needs to be simultaneously recorded on a plurality of exposed objects (photosensitive drums). Thus, laser diodes for emitting a plurality of light beams having different wavelengths need to be simultaneously driven to emit light.
However, when two adjacent laser diodes formed on the same substrate, as shown in FIGS. 1 and 2, are simultaneously driven, due to the effect of cross-talk of the adjacent laser diodes, a light-emitting characteristic deteriorates due to a variations a material for a light-emitting portion caused by heating or a variation in resistance value of a circuit such as an electrode.
Accordingly, when a composite light source having a plurality of laser diodes is used as a light source of a printer, if the plurality of laser diodes are simultaneously driven and emit light, printing quality deteriorates.